Bug found by Yunqing relating to the correction for size at 8K and
above in get_twopass_worst_quality().

The basis for the correction was changed to the linear size relative to
1080P as a baseline and the adjustment has been clamped to prevent
problems at extreme images sizes.

For 1080P the results on our test sets were neutral but the low res and
mid res sets saw a small gain (0.1%-0.2% average).

I would also expect some gains on 4k and larger content where the
previous correction was overly aggressive.

Change-Id: I30b026b5f4535e9601e3178d738066459d19c8fb